Sql server statistics not updating datingtime com

Posted by / 30-Sep-2019 14:59

Sql server statistics not updating

Density information is calculated for each set of columns, forming a prefix in the statistics object.Filtered statistics, on the other hand, are not created automatically by the query optimizer, but only when a filtered index is created or when a CREATE STATISTICS statement with a WHERE clause is issued.Using WITH FULLSCAN or using a larger sample can be of benefit, especially with data that is not randomly distributed throughout the table.Scanning the entire table will naturally give you the most accurate statistics possible.If an execution plan for a specific query exists in the plan cache and the statistics that were used to build the plan are now out of date, then the plan is discarded, the statistics are updated, and a new plan is created.In a similar way, updating statistics, either manually or automatically, invalidates any existing execution plan that used those statistics, and will cause a new optimization the next time the query is executed.When it comes to determining the quality of your statistics, a fact to consider is the size of the sample of the target table used to calculate said statistics.The query optimizer determines a statistically significant sample by default when it creates or updates statistics, and the minimum sample size is 8MB (1,024 pages) or the size of the table if it’s smaller than 8MB.

SQL Server defines when statistics are out of date by using column modification counters, or colmodctrs, which count the total number of modifications for the leading statistics column since the last time statistics were updated.

However, if the query optimizer needs to automatically update these index statistics, it has to go back to a default sample because it may take too long to scan the entire table again.

By default, SQL Server needs to wait for the update statistics operation to complete before optimizing and executing the query; that is, statistics are updated synchronously.

Consider that if statistics are built after scanning 50 percent of a table, then SQL Server will assume that the 50 percent of data that it has not seen is statistically exactly the same as the 50 percent it has seen.

In fact, given that statistics are always created alongside a new index, and given that this operation scans the entire table anyway, index statistics are initially created with the equivalent of the WITH FULLSCAN option.

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Basically, for tables bigger than 500 rows, a statistics object is considered out of date if the colmodctr value of the leading column has changed by more than 500 plus 20 percent of the number of rows in the table.